Short wavelength devices (e.g. those sensitive to wavelengths in the near infrared, visible or ultraviolet range below 1.5 .mu.m) are relatively common. Because these devices do not require narrow bandgap materials, it is possible to use materials which are well known and easily integratable. To detect longer wavelength (lower energy) radiation with wavelengths above 7 .mu.m such as infrared radiation, it is necessary to use either narrow bandgap materials, wider bandgap materials in which material is strained to narrow the bandgap or specialized structures which simulate the absorption properties of narrow band materials. The detection frequency or detection wavelength is the frequency or wavelength of the radiation which the detector is designed or tuned to detect (i.e., the frequency or wavelength which generates desirable photocurrent in the device).
As an example of an avalanche detector which operates on the principle of interband absorption in wide bandgap materials see U.S. Pat. No. 4,203,124 which describes a multi p-n junction device adapted to detect wavelengths in the 1.0 to 1.5 .mu.m spectrum. Detection is accomplished by interband absorption. U.S. Pat. No. 4,722,907 describes an avalanche photodetector (APD) device adapted to detect wavelengths in the 1.0-1.5 near infrared spectrum. This device also works by interband absorption in wide bandgap materials and includes superlattice avalanche multiplication control. As an example of a p-i-n strained layer superlattice photodiode adapted to detect wavelengths in the visible spectrum see U.S. Pat. No. 4,616,241. This device operates on the principle of interband absorption in wide bandgap materials by strained-layer superlattices.
In the prior art, long-wavelength photodetectors, i.e., detectors adapted to detect wavelengths in the infrared (greater than 7 .mu.m) spectrum, relied primarily upon either interband absorption in narrow bandgap materials, interband absorption by strained-layer superlattices, or intersubband absorption in doped superlattices or multi-quantum-well detectors as the primary detection mechanism.
As an example of a long wavelength detector which operates on the principle of intraband absorption in doped superlattices or multi-quantum well detectors see U.S. Pat. No. 4,711,857. Rather, than relying on intersubband absorption, this device detects incident radiation by photoexcitation of majority carriers. Current flows when the incident radiation is sufficient to excite carriers with sufficient energy to exceed the quantum barriers.
Another device for the detection of long wavelength radiation is a metal-semiconductor interface device which operates on the principle of internal photoemission. This is a majority carrier device. See, for example, "Optically Enhanced Schottky Barrier Photodetectors for IR Image Sensor Application," by H. Pohlack in Phys. Stat. Sol., (a) 97, pp. K211-K215 (1986).
Interband absorption devices utilize narrow bandgap materials such as HgCdTe. The use of interband absorption is an old idea and fairly mature. Its primary drawbacks are the poor material properties of narrow bandgap materials such as the HgCdTe family, and the fact that these materials are extremely difficult to use in fabricating arrays which can be addressed or integrated with other electronic devices. These are minority carrier devices. See, for example, "High-Performance Photovoltaic Infrared Devices in Hg.sub.1-x Cd.sub.x Te on GaAs," by E. R. Gertner, S. H. Shin, D. D. Edwall, L. O. Bubulac, D. S. Lo and W. E. Tennant in Appl. Phys. Let., Volume 46, No. 9, pp. 851-853 (May 1, 1985).
Intersubband absorption (discussed in detail herein below) devices are relatively new and may be constructed using, for example, a GaAs-AlGaAs multi-quantum-well structure. These devices rely on optically active superlattice structures to detect incident radiation of an appropriate wavelength. These are majority carrier devices.
In long wavelength (e.g., greater than 7 .mu.m) detectors, doped superlattice structures function effectively as detectors and have advantages over interband absorption devices where integration of devices is important. Integration is difficult on other types of prior art devices because of the materials processing problems encountered. In addition to materials problems, the drawbacks to using most prior art detectors, including doped superlattice structures, include large dark currents and a lack of voltage tunability. An additional problem in some doped superlattice structures is the very small solid angle of detection. Those devices that rely on intersubband absorption must be oriented at a specific angle to the incident radiation to increase their sensitivity. Additionally, devices which rely on minority carrier conduction tend to be slower than those which rely on majority carrier condition.
Of all the problems listed above, the problem of dark current is the most troubling. The large dark currents characteristic of prior art detectors are primarily a result of thermal energy in the device. In most of these prior art devices the magnitude of dark current makes it difficult to distinguish it from the desired photo current. Thus dark current substantially limits the sensitivity of these devices. This problem may be aggravated by the avalanche effect. Because the primary component of dark current is thermal excitation of the electrons into a conduction state, and because no effective means has been found to filter the dark current component, the primary method of controlling dark current is by lowering the thermal energy. One known way to lower the thermal energy and thus limit dark current is to cool the detector to cryogenic temperatures. Cooling reduces thermal excitation of carriers and the current generated thereby. However, cryogenic cooling is expensive and requires complicated heat transfer apparatus.
Another problem with most prior art devices is that the detection frequency is a function of the device structure and materials characteristics. Therefore, the detection frequency is fixed once the device is manufactured and cannot be changed by, for example, adjusting the bias voltage. Most long wavelength detectors are constrained by their structure and detection mechanism to operate at a fixed detection frequency and are, therefore, not tunable. One exception is the n-i-p-i structure (currently used for less than 1.0 .mu.m applications) which is voltage-tunable. See, for example, "Tunable Absorption Coefficient in GaAs Doping Superlattices," by G. H. Dohler, H. Kunzel and K. Ploog in Physical Review B, Vol. 25, No. 4pp. 2616-2626 (Feb. 15, 1982). Since the detection mechanism in the n-i-p-i structure is indirect real space transitions between the conduction and valence band, it may be tuned by changing its bias voltage. Changing the bias voltage across the device changes the energy separation between the valence and conduction band of neighboring doped regions. However, this type of device is not known to be adaptable to long wavelength applications.
Thus it would be advantageous to provide a fast, integratable, highly sensitive, long wavelength detector which filters out a substantial portion of the undesirable dark current without substantially attenuating the desirable photocurrent. In addition, it would be advantageous to provide a long wavelength detector which is electrically tunable over a range of detection frequencies. It would also be advantageous to provide a long wavelength detector which functions over a large solid angle of incident radiation, including orthogonal radiation. Finally, it would be advantageous to provide an integratable, highly sensitive, long wavelength detector which operates by majority carrier conduction.
A GaAs-AlGaAs doped superlattice structure which operates on the principle of intersubband absorption is described by Levine et al. in their paper entitled "New 10 .mu.m Infrared Detector using Intersubband Absorption in Resonant Tunneling GaAlAs Superlattices," B. F. Levine et al., Appl. Phys. Lett. 50 (16), Apr. 20, 1987. This detector is intended to overcome the problems associated with integration of long wavelength detectors with electronic circuits by using a doped Al.sub.x Ga.sub.1-x As superlattice structure for detection which relies on intersubband absorption. According to the article, the Levine et al. detector is intended to achieve long wavelength (10 .mu.m) infrared sensitivity using Group III-V semiconductor materials, rather than the less technologically advanced Hg.sub.x Cd.sub.1-x Te alloys. In the Levine device, the doping in the cathode and anode is necessarily limited to prevent absorption of light and large dark currents. What doping there is facilitates current transport to and from the device. Thus, the optically active component of the Levind et al. device is the doped, superlattice portion in which the photocurrent results from intersubband excitation of free carriers.
However, intersubband absorption has inherent drawbacks which prevent the Leving et al. device from achieving optimum results. First, because of the principle of operation of this device, it must be biased to a voltage that is close to that which causes avalanche breakdown, a condition in which the electric field causes multiplication of thermally generated dark current carriers, before efficient photodetection occurs. Thus, this device has relatively large "dark current," i.e., noise, when biased in its optimal operating condition. Second, since the intersubband absorption vanishes if light is normally incident on the layers forming the quantum wells, the detector must be oriented at Brewster's angle to maximize the sensitivity. Thus, this detection scheme is only sensitive over a very limited solid angle.
FIG. 1 illustrates an unbiased two-well quantum lattice device including an anode 10, cathode 12, quantum wells 14 and barrier regions 16, which will be described for convenience. E.sub.f is the Fermi energy level of cathode 12. E.sub.c is the conduction band energy level. The quantum wells 14 of prior-art intersubband absorption devices (e.g. the Levine et al. device), while they could be constructed of essentially the same materials as the present invention, are doped to provide electrons in (at least) the lowest energy quantum-well subband 18 (henceforth referred to as lower subband). Higher energy subbands depleted of free carriers 20 are also included. Further, barrier regions 16 are thick enough to prevent substantially all elelctron tunneling.
When a doped superlattice device is biased as in FIG. 2, an electron excited from the lower subband 18 of a quantum well to the higher energy subband 20 will tunnel more easily through the device because the width of the barrier 16 is decreased by the bias slope 24 and the upper subband is closer to the top of the barrier. The bias slope 24 is induced by placing a DC potential across the device. Therefore, when an electron is excited from the lower subband 18 to the upper subband 20 (e.g., by irradiating the device), it travels easily from the quantum wells to the anode. As the electrons travel from one quantum well to the next, they can create an avalanche effect (especially if the device is biased to enhance avalanche), increasing the current in the device exponentially.
FIG. 3 illustrates intersubband absorption wherein added energy (e.g., photon energy in the form of incident radiation) causes an electron to jump from the lowest 18 to a higher energy subband 20, allowing the electron to escape from the quantum well 14 and causing a current to flow.
Therefore, to induce current flow, energy added to the device, for example, by irradiating it, must be sufficient to cause the electrons to jump from the lowest energy subband to a higher energy subband. Therefore, the photon energy of the incident radiation must equal the energy required to elevate an electron from one subband to the next. As radiant energy travels through the device, photons are absorbed in each of the quantum wells, causing each quantum well to contribute to the device current. More specifically, when light of an appropriate wavelength passes through the device, it gives up energy to the electrons in each quantum well. Thus, each quantum well creates its own current, which can be amplified by an avalanche effect. This is an example of intersubband absorption in doped multi-quantum-well detectors. In these devices it is the quantum well structure which is photo-sensitive and detects the incident radiation.
It will be noted that the response of an intersubband detector and of most other long wavelength detectors is not symmetrical around the detection frequency. That is, while it may be very responsive to incident radiation with frequencies above the frequency of interest, its response falls off rapidly at frequencies below the frequency of interest. Thus, broadband or shorter wavelength incident radiation may include wavelengths below the detection frequency with photon energies sufficient to cause the electrons in the lower subband to escape over the barrier layers, causing an undersirable photo current to flow. This higher frequency (higher energy) radiation may be filtered before striking the detector (e.g., by placing a filter with a cut-off frequency at approximately the detection frequency in front of the detector). Radiation with frequency below the detection frequency is filtered by the device because the photon energy is not sufficient to enable an electron to jump from the doped subband over the barrier and, therefore, does not induce current flow. It will be noted that it may be difficult to filter the frequencies immediately above the detection frequency without reducing the detector's sensitivity to wavelengths at the detection frequency. Further, it is extremely difficult to effectively filter shorter wavelength radiation while tuning the detector to adjust the detection frequency.
In addition to the desirable current induced in the detector by incident radiation, undesirable currents are also induced by various mechanisms. In all prior art devices undesirable current components may be developed in any of the active regions (e.g., in the doped quantum wells) by extraneous energy sources (primarily thermal and electric field sources) even in the absence of incident radiation. This undesirable current is referred to as dark current. In many of these devices the undesirable dark current could be multiplied and avalanched. Further, these devices are not adapted to limit the dark current to any specific energy level (e.g., the photon energy of the detection frequency), resulting in a large, broad band dark current which reduces the device's sensitivity. Thus, for the detector to be useful, the added energy due to incident radiation at the detection frequency must be sufficient to be detected or extracted from the dark current.
A significant dark current may also result from resonant tunneling. In a biased doped superlattice, where the quantum wells are doped and thus contain free carriers (e.g. electrons or holes), alignment of the quantum well subbands, as in FIG. 4, will result in a resonant tunneling current. The level of dark current and the quantum efficiency of the detector increase with doping level in the quantum wells, which is also proportional to the detector sensitivity. Thus, the more sensitive the detector, the greater the dark current problem.
A similar phenomenon occurs where the cathode is doped and, because of the bias, the Fermi level of the cathode meets or exceeds the aligned subbands. This will cause the free carriers in the cathode to flow as current. Thus, in doped superlattice detectors resonant tunneling is an undesirable effect which adds to the dark current. FIG. 5 illustrates an intersubband absorption device with a large doping concentration in the cathode which is biased to ensure resonant tunneling.
A significant disadvantage of some doped superlattice structures is the limited solid angle of detection. In doped superlattice structures relying on intersubband absorption for detection, it is necessary for optimal operation that the incident light hit the device at an appropriate angle (Brewster's angle). Otherwise, the incident radiation will not give up photon energy to each quantum well. If the radiation does not hit the device at Brewster's angle or some small variation thereof (e.g., if it enters the detector orthogonal to the quantum wells) the electric field (the E field) of the incident radiation will be perpendicular to the matrix elements. Thus, the energy component required to boost the electrons from the lowest subband energy to higher energy subbands is zero (i.e., there is no intersubband absorption in the quantum wells).
In a doped superlattice relying on intersubband absorption, the level of doping in the cathode must be limited since it is normally composed of the same material as the quantum wells. Doping the cathode causes its Fermi level to rise to a point where current could flow without photoexcitation, reducing the device's sensitivity substantially. This can be seen in FIG. 4 where the Fermi level is only slightly above the lowest subband of the cathode. Introducing any further doping as in FIG. 5 would raise the Fermi level to the point where resonant current may flow continuously.